ICMCTF2014 Session C4-1: Thin Films for Energy Related Applications

Quantum dot (QD) superlattice incorporated in the active region of a p-i-n single-junction solar cell can be used as a potential means of utilizing sub-bandgap infra-red photons to generate additional photocurrent, through absorption via superlattice miniband states, beyond that corresponding to the valence-to-conduction (VB-CB) band transitions.

Proposed implementation of QD-IB solar cell must be accompanied by two-step carrier generation via IB states, but it has been difficult to clearly demonstrate this concept at room temperature [2,3]. The demonstration of QD-IB solar cell is presently undergoing two research stages. The first is to develop technologies to fabricate high-density QDs array or superlattice with low interface defect densities at the heterointerfaces. The fabrication of QDs array is most commonly achieved by taking advantage of spontaneous self-assembly of coherent 3-dimensional islands in lattice-mismatched epitaxy, long known as Stranski-Krastanov (S-K) growth. The second stage is to realize partially or half-filled IB states in order to ensure an efficient pumping of electrons by providing both the empty states to receive electrons being photo-excited from VB, and filled states to promote electrons to CB via absorption of second sub-bandgap photons.

[1] A. Luque and A. Martí, Phys. Rev. Lett. 78 (1997) 5014. [2] A. Martí et al, Phys. Rev. Lett. 97 (2006) 247701. [3] Y. Okada et al, J. Appl. Phys. 109 (2011) 024301.

The synthesis of CZTS thin films conventionally consists in a “two-step” process: metallic precursors are deposited by pulsed laser deposition, RF sputtering, electro-deposition or sol gel methods, and then thin films are annealed in presence of sulfur gas [2]. For large scale applications, it is necessary to develop a “one-step” process. Reactive magnetron sputtering, which is widely used in the coating industry, allows a good control of the plasma parameters, thin film growth conditions and thus film characteristics. Keeping in mind that the efficiency of the final solar cell strongly depends on the cristallinity of the CZTS absorber layer, reactive magnetron sputtering is a relevant technique for the growth of solar cell absorbers.

In this work CZTS films were synthesized by reactive magnetron co-sputtering and a particular attention was given to the influence of the experimental parameters (powers, pressure, gas mixture) on the thin film properties (micro-structure and composition).

CZTS thin films were deposited on Mo-coated soda lime glass substrates. H2S was used as reactive gas. Two metallic targets, zinc and copper-tin alloy were sputtered simultaneously in DC and pulsed DC mode, respectively. Experimental parameters such as the powers applied to the metallic targets, the total pressure, the H2S/Ar flow ratio were varied. The crystalline constitution, the chemistry as well as the microstructure of the deposited layers are evaluated by XRD, XPS and SEM, respectively.

A pure phase of kesterite CZTS was obtain at 5 mtorr, 10 sccm of each gas and a ratio of powers (CuSn/Zn) equal to 0.4. The increase of both the pressure and the gas flow rate was found to promote the synthesis of films composed on wurtzite and stannite CZTS phases with the presence of impurity phases such as Cu2SnS3, Cu2-xS, SnS and ZnS.

References

[1] H. Wang, International Journal of Photoenergy (2011)

[2] F. Liu, Y. Li, K. Zhang, B. Wang, C. Yan, Y. Lai, Z. Zhang, J. Li, and Y. Liu, Solar Energy Materials and Solar Cells, 94 (2010) 2431-2434

Cu₂ZnSnS₄ (CZTS) thin film solar cell is a low cost, environmental harmless solar cell. CZTS is a I₂–II–IV–VI₄ quarternary compound semiconductor. All the elements in CZTS compound are abundant on earth and non-toxic.

Up to now, several techniques have been used to fabricate CZTS thin films, including vacuum and non-vacuum methods. Vacuum methods include thermal co-evaporation, sputter deposition, pulsed laser deposition, and etc. Non-vacuum methods include electrodeposition and solution processes. Among the various methods, the electrodeposition followed by sulfurization method has been found to be a low-cost and simple way to fabricate CZTS thin films.

In this research, various precursor films containing Cu, Zn, Sn, and S were first obtained by the electrodeposition of Cu , Sn, Zn, and ZnS in sequence on Mo coated glass substrates. The Sn and the ZnS layers were deposited under pulsed powers. For depositing the Sn layer, the current was pulsed while the voltage was pulsed for depositing the ZnS layer. The pulse frequency was varied when depositing ZnS. The deposition of ZnS was aimed to enhance the film quality, anticipating to receive the same advantage, i.e., improved conversion efficiency, as the precursor films obtained using sputter deposition. Before, CZTS precursor films were made by sputtering Cu, Sn, and ZnS (instead of Cu, Sn, and Zn). In doing so, the conversion efficiency of the CZTS films increased. If using Zn/Sn/Cu/Mo stacking sequence as the precursor layer ,the volume will become three times large after sulfurization, which may lead to defect in the CZTS film. Then, the quality of the CZTS film is reduced and the conversion efficiency is low compared to the ZnS/Zn/Sn/Cu/Mo sequence. So it is considered that using electrodepositing ZnS on Zn/Sn/Cu/Mo can also lead to the same advantage and reduce the cost of production. The obtained electrodeposited samples were then subjected to sulfurization in H₂S at 500℃ to 550℃ for 60 to 120 minutes. The as-deposited samples and the sulfurized samples were characterized using scanning electron microscope, X-ray diffraction, atomic force microscopy, Raman spectroscopy, ultraviolet-visible spectroscopy and Hall measurement .

Photovoltaic (PV) energy, concerned as the most abundant, clean and sustainable of all the renewable energy resources, is one of the most promising alternative solutions for the escalating demand of energy until date. It has reached a turning point for further penetrating the usage of various photovoltaic devices due mainly to the material and production costs. All photovoltaic devices practically combine a p-n junction, where one side as the absorber material ejects e-hole pairs, namely, electricity. The absorber layer can be organic material, inorganic material and/or the combination, while the approaches for manufacturing the absorber layer either in thin-film form or in bulk material have been paid much attention these days, all aiming to reduce material cost and production cost.

The intrinsic p-type cubic structure titanium monoxide (TiO) with suitable band gap energy, reasonable raw material cost and particularly simple stoichiometry has been chosen as a potential candidate in microelectronic applications and may have the opportunity to use as a p-type absorber layer for photovoltaic purpose. Among various printing technologies recently explored to fabricate solar cell layers, inkjet printing technology is really promising by virtue of the compatibility with distinct substrates and the non-contact ability with the substrate surface, leading to precisely patterned areas when using drop on demand (DOD) technology. Therefore, this work aims to fabricate TiO film by using inkjet printing technique.

Importantly, utmost efforts were made to adjust parameters when preparing TiO ink for optimal printability. After finishing ink-jet printing work, post-annealing treatment was carried out for obtaining final dense layer. Microstructure, optical and electrical properties of the printed TiO layer were revealed so as to evaluate the feasibility of using ink-jet printed TiO layer for photovoltaic system application.

Pd-Ag thin films were deposited by magnetron sputtering onto tubular alumina supports for hydrogen selective separation and purification purposes. The thin film columnar microstructure dictates the permeation fluxes through the membrane. From scanning electron microscopy analysis it was observed that different sputtering deposition pressures can lead to distinct columnar structure growth. X-ray diffraction patterns provided evidence of a Pd-Ag solid solution whose crystalline texture can be altered by the deposition pressure. The gas-permeation results, involving both nitrogen and hydrogen, have shown that the Pd-Ag membrane supported on porous Al₂O₃ is selective towards H₂, which demonstrates that the membrane has a reasonable capacity to hinder the permeation of this gas, providing only catalytic diffusion of atomic hydrogen. The selectivity factor α (H₂/N₂) obtained for the optimized membranes was studied as a function of temperature.

LiNi_0.5Mn_1.5O₄ (LNMO) thin films on stainless steel sheets have been synthesized using radio frequency magnetron sputter deposition followed by thermal anneal in atmosphere. Various negative biases were applied on the substrate during deposition. The structural evolution of LNMO thin films under different negative biases have been characterized by Raman spectroscopy and X-ray diffraction. The films under various biases exhibit crystalline spinel structure with ordered and disordered phases. The results indicate that the content of disordered phase LNMO increases with increasing negative bias. The electrochemical properties of LNMO thin films as cathode materials for lithium ion batteries were investigated. Two distinctive regions around 4.7 V and 4 V can be found in the discharge curves, corresponding to the reactions of ordered phase and disordered phase, respectively. The capacity of LNMO thin film electrodes under suitable negative bias can be optimized.